This invention relates to a magnetic recording medium, more particularly, to a magnetic recording medium having a topcoat layer exhibiting improved running stability and durability.
In recent years, magnetic recording media were widely used in various spheres including audio, video, computer, and magnetic disk applications. The advanced magnetic recording media are required to be more compact while it is desired to record the increasing quantity of information therein. There is the strong need for increasing the recording density of magnetic recording media.
To meet such a need, binder free magnetic recording media, particularly those magnetic recording media having a magnetic layer in the form of a ferromagnetic metal thin film are more advantageous than conventional coating type magnetic recording media because the former can be made more thinner because of the absence of binder and has greater saturation magnetization. However, a problem arises when magnetic recording media are moved in sliding contact with a magnetic head as in the operation of recording, reproducing, and erasing magnetic signals. Since the magnetic layer of the binder free magnetic recording media is formed by electroplating, electroless plating, sputtering, vacuum deposition, ion plating or similar techniques without a binder, the magnetic layer is relatively readily abraded away or broken by friction due to the high speed relative motion with the magnetic head. The binder free magnetic recording media have another problem that their magnetic layer is liable to corrosion at the surface. A progress of corrosion reduces the useful properties of magnetic layer such as head contact and abrasion resistance, adversely affecting electromagnetic properties.
There have been proposed a number of means for mitigating impact and friction to the magnetic laYer of magnetic recording media. One method is by applying a lubricant on the surface of magnetic layer (see Japanese Patent Publication No. 39-25246)
Japanese Patent Publication No. 57-29769 proposes to form a lubricating layer (or a backcoat layer) based on a liquid or semi-solid lubricant and an organic binder on the surface of magnetic recording medium opposite to its magnetic layer for the purpose of continuously providing lubrication to the magnetic layer surface. It is described in the publication that the lubricant migrates from the backcoat layer to the adjoining surface of the magnetic layer of the magnetic recording medium wound in a roll form, and the lubricant is thus supplied to the magnetic layer surface, thereby increasing the durability or resistance to mar and peeling of the magnetic layer and accomodating variations in coefficient of dynamic friction
However, in the case of the lubricant coating described in Japanese Patent Publication No. 39-25246, lubrication does not last for a sufficient period because the lubricant is removed by a magnetic head and guide pins. Thus no improvements in rust prevention and durability are expectable.
In Japanese Patent Publication No. 57-29769, lubricant is contained in the backcoat layer only without forming a topcoat layer on the surface of the magnetic layer Then the friction between the magnetic layer surface and a magnetic head is still so great as to lead to running troubles. No satisfactory results are obtained with respect to corrosion resistance or rust prevention. If the lubricant in the backcoat layer is transferred to the magnetic layer uncovered with a topcoat layer, magnetic metal films become unstable, causing head clogging, output reduction, and even reproduction of no image. Sometimes, friction resistance is not fully reduced and even the film is abraded away or broken. This is true for most of current magnetic metal films, that is, oxygen-containing metal films although not outstanding when oxygen is not introduced in deposited films (that is, oxygen-free metal films, see Japanese Patent Publication No. 57-29769).
On the basis of the concept of containing lubricant in the backcoat layer, it would readily occur to those skilled in the art to apply lubricant to the topcoat layer. The coating of the topcoat with lubricant temporarily reduces friction, but the effect does not last. The reslting magnetic recording medium has weaknesses in rust prevention, corrosion resistance, and durability.